Rather than having to assume that we only ever produce one
artifact, have each upload target contain the filename it expects
to upload from the osbuild output.
An image file is always explicitly named in the manifest, and we
leave it up to each distro to decide how this is done, but the
convention is to use the same image filename as used when
downloading the image through weldr.
Now make this policy explicit, by quering the distro for the image
name and inserting it into each upload target.
Signed-off-by: Tom Gundersen <teg@jklm.no>
